(a) Field of the Invention
The present invention relates to a forward-oblique viewing optical system which is to be applied to long and slender image-transmitting optical systems such as endoscopes and for which the observing direction is at a pre-determined angle in respect to the image-transmitting direction (longitudinal direction of the endoscope).
(B) Description of the Prior Art
An optical system in a distal end of a long and slender image-transmitting optical system is generally required to be small in size especially in its outer diameter. This is due to the following reasons. That is, if an outer diameter of an endoscope, for example, is larger, the patient has to feel severe pain at the time of inserting. Moreover, the outer diameter will be physically limited by the portion to be observed and it will becomes sometimes impossible to insert the distal end when its diameter is large. For a cystoscope, for example, the outer diameter of the distal end is required to be very small, i.e., 2 mm to 3 mm.
For optical systems to be arranged in the distal end of the diameter limited as above and to be used for purposes other than forward viewing, said purposes can be attained by employing a right-triangular prism of 45.degree. or 30.degree. in case of side viewing when the observing direction is at 90.degree. in respect to the longitudinal direction of the endoscope or in case of forward-oblique viewing when the observing direction is at 60.degree.. In case of forward-oblique viewing when the observing direction is at 30.degree. from the forward-viewing direction, however, it is unavoidable to arrange the optical system as shown in FIG. 1A and FIG. 1B. That is, as shown in FIG. 1A or FIG. 1B, it is necessary to arrange a prism P.sub.1 or P.sub.2 of special shape in front of the objective O of the endoscope in order to change the direction of rays from the object to be observed, which is in forward-oblique direction in respect to the optical axis of the objective 0, into the direction along the O, axis of the objective O. Reference symbol I.G. in said figures represent an image-transmitting optical element such as image fiber which is arranged according to known art so that an image of the object will be focused on its end face by means of the objective. When, however, a prism of special shape as mentioned in the above is adopted, the length represented by reference symbol h in said figures becomes large and, therefore, it becomes impossible to satisfy the above-mentioned requirement to make the diameter of the distal end small. If the outer diameter is made small in case of the optical system employing a prism of special shape as shown in FIG. 1A and 1B in order to satisfy said requirement, it becomes impossible to assure sufficient effective diameters on entrance and exit surfaces.